1. Field of the Invention
This invention relates generally to read heads for patterned magnetic recording media, such as disks for use in magnetic recording hard disk drives, and more particularly to a read head capable of reading multiple data tracks from the patterned medium.
2. Background of the Invention
One type of proposed patterned magnetic recording media is referred to as “bit-patterned media” (BPM). In a BPM disk, the magnetizable material on the disk is patterned into discrete concentric data tracks, with each track patterned into small isolated data islands such that there is a single magnetic domain in each island or “bit”. The single magnetic domains can be a single grain or consist of a few strongly coupled grains that switch magnetic states in concert as a single magnetic volume. This is in contrast to conventional “continuous media” disks wherein a single “bit” may have multiple magnetic domains separated by domain walls. To produce the required magnetic isolation of the patterned islands, the magnetic moment of the spaces between the islands must be destroyed or substantially reduced so as to render these spaces essentially nonmagnetic. In one type of BPM disk, the data islands are elevated, spaced-apart pillars that are separated by nonmagnetic trenches or recesses.
Another type of proposed patterned magnetic recording media is referred to as “discrete-track media” (DTM). In a DTM disk, the magnetizable material on the disk is also patterned into discrete concentric data tracks. However, unlike a BPM disk the tracks in a DTM disk are not patterned into isolated data islands but are formed as continuous magnetizable material. In a DTM disk, the data tracks are typically elevated lands that contain magnetic material. Trenches or grooves that are recessed below the elevated lands serve as nonmagnetic guard bands between the tracks. The nonmagnetic guard bands are either formed of nonmagnetic material or contain magnetic material but are recessed far enough below the elevated data tracks to not adversely affect the readback signals from the data tracks.
The data in patterned-media BPM and DTM disks can be read back by a conventional magnetoresistive (MR) sensor or read head like that used in conventional continuous-media magnetic recording disk drives. One type of MR read head is based on the giant magnetoresistance (GMR) effect. A GMR sensor has a stack of layers that includes two ferromagnetic layers separated by a nonmagnetic electrically conductive spacer layer, which is typically copper (Cu). In one type of GMR sensor, called a “spin-valve”, one of the ferromagnetic layers has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer has its magnetization direction “free” to rotate in the presence of an external magnetic field. With a sense current applied to the sensor and in the presence of an applied magnetic field, the rotation of the free-layer magnetization relative to the pinned-layer magnetization is detectable as a change in electrical resistance.
In a magnetic recording disk drive spin-valve read sensor or head, the stack of layers are located in the read “gap” between magnetic shields. The magnetization of the pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance. If the sense current flowing through the stack is directed parallel to the planes of the layers in the sensor stack, the sensor is referred to as a current-in-the-plane (CIP) sensor, while if the sense current is directed perpendicular to the planes of the layers in the sensor stack, it is referred to as current-perpendicular-to-the-plane (CPP) sensor.
A CPP-GMR spin-valve type of read head is one type of CPP MR sensor or read head. Another type of CPP MR sensor or read head is a magnetic tunnel junction sensor, also called a tunneling magnetoresistance or TMR sensor. While in a CPP-GMR spin-valve type of read head the spacer layer is formed of an electrically conductive material, such as Cu or other metal or metal alloy, the spacer layer in a CPP-TMR type read head is formed of a thin electrically insulating material, such as TiO2, MgO or Al2O3. Like a CPP-GMR spin-valve type of read head, a CPP-TMR read head also has one of the ferromagnetic layers with its magnetization direction pinned and the other ferromagnetic layer with its magnetization direction free to rotate in the presence of an applied magnetic field from data recorded on the disk. The resistance of a GMR or TMR sensor depends on the relative orientation of the magnetizations in the two ferromagnetic layers.
With the proposed use of BPM and DTM disks in disk drives there is an opportunity to implement improved CPP read heads that take advantage of the fact that the data tracks and data bits are precisely defined and located on the disks. This makes possible the reading and writing of multiple data tracks. The ability to read back multiple data tracks enables a higher data rate and new applications like high-resolution streaming video and high-speed databases without increasing the linear bit density on the disks or the RPM of the disk drives.